This invention relates to the field of optics, and more particularly to a method of making color and profile measurements on three-dimensional surfaces.
The ability to measure the color and three-dimensional shape of objects at a high-resolution allows the input of representations of real objects into computers. These digital models can then be stored, transmitted, analyzed and displayed. For example artifacts from museum collections can be digitized and stored as models, which can be made available for interactive viewing, cataloguing and analysis by curators, art historians or the public. Such applications require high-resolution models constructed from accurate measurement of geometric and color properties of the object""s surface.
One technique for carrying out such measurements is employed in the National Research Council of Canada""s laser range sensor. This includes a device for scanning a surface with an incident laser beam to form a reflected beam, which is directed onto a photosensitive sensor linear array, typically a CCD (Charge Coupled Device). In the monochromatic version, the relative position of the reflected beam on the sensor array as the incident beam is scanned across the target surface gives profile information by triangulation. The amount of light returned to the sensor array can also be measured and used to estimate the reflectance of the target surface. In the polychromatic version, color information is obtained in addition to profile information by using a polychromatic laser and splitting the reflected light into the three component wavelengths, and imaging them at different locations on the CCD array. The output signal of the CCD array will contain peaks corresponding to the image of the spot for each laser wavelength. The position of the target surface can be obtained by triangulation, assuming that the identity of each peak, that is its original wavelength, is known. The amount of light returned to the sensor for each wavelength can be calculated from the shape of the peak. The intensity of the primary wavelengths on the sensor array permits the color of the surface to be calculated knowing the amount of each wavelength present in the reflected beam. Such a technique is described in, for example, U.S. Pat. No. 5,177,556.
A significant drawback of this method is its inability to identify the original wavelength of a peak on the CCD when only one wavelength is reflected, thus preventing profile estimation at that point. A wrong identification of wavelength leads to a gross error in depth estimation as well as in color measurement.
Another known technique for color measurement uses an auxiliary photodetector cell responsive to each of the three wavelengths to analyze a portion of the light returned to the sensor in order to extract the color information. This method gives a non-ambiguous measurement, but loses one of the advantages of the first technique, which is that the estimation of the returned intensity can be precisely limited to the identified peak corresponding to the portion of the surface profile illuminated by the laser spot. The photodetectors are sensitive to the entire instantaneous field of view and can be easily contaminated by ambient light or multiple reflections. Ambient light outside the wavelengths of the laser can be eliminated with the aid of filters. However, residual illumination remains (depending on the spectral content of the ambient light) or indirect reflections from the laser source coming from elsewhere than the surface illuminated by the light spot may also influence the measurements. Such a method is described in U.S. Pat. No. 5,708,498.
In the first-mentioned technique, the sensor provides depth and color information at a similar rate, since the depth is determined from the position of one or a combination of the peaks imaged on the CCD, and the color is derived from the amplitude of each peak""s distribution. At each measurement cycle, the signal produced by the CCD is analyzed to extract position and amplitude of the peaks.
A still further drawback is that the acquisition of three-dimensional data in regions where the reflectivity suddenly changes results in errors in the profile measurements.
A well-known difficulty common to all triangulation-based laser sensing systems is that sudden changes in reflectance of the measured surface are likely to cause errors in profile measurement. This occurs because the measurement is derived from imaging a finite area illuminated by the laser. If the underlying surface exhibits a change in reflectance, then the image of the spot will be skewed as compared to a uniform surface. The position measurement on the CCD may be influenced and lead to a bias in estimating the shape of the profile.
An object of the present invention is to alleviate the aforementioned disadvantages of the prior art.
According to a first aspect of the present invention there is provided a method of determining the color and profile of a target surface, comprising the steps of scanning the target surface with an incident light beam containing a plurality of component wavelengths; forming a beam of light reflected from said target surface; forming the reflected beam into one or more separate sub-beams corresponding to the component wavelength or wavelengths of the reflected beam; directing said one or more sub-beams onto a sensor array; detecting the positions of said one or more sub-beams on said sensor array as said incident beam moves over the target surface; directing at least a portion of the reflected beam onto wavelength sensitive photodetector means to obtain data representative of the approximate wavelength composition of said reflected beam; and determining the color and profile of the target surface from the relative positions and shapes of peaks produced by said one or more sub-beams on said sensor array using said data representative of the approximate wavelength composition to resolve potential ambiguities in the results obtained from said sensor array.
The proposed color measurement apparatus is a hybrid system that uses both color-separated peak measurement on the CCD and auxiliary measurement with a photodetector. Unlike the prior art, the advantages of color measurement obtainable with the sensor array are preserved, with the auxiliary photodetector serving mainly to resolve ambiguities in the precise measurements obtainable by the CCD array. The photodetector should preferably be sampled at a rate higher than the CCD for reasons that will be explained in more detail below.
The most significant advantage of such a configuration is that it permits the surface color to be determined when, for example, only a single reflected wavelength is present. The photodetector cell, though affected by light sources other than the region illuminated by the laser spot, will give a measurement dominated by one or more wavelengths present on the surface that is directly illuminated, and will therefore permit the identification of the wavelength beam illuminating the CCD when only one beam is present. In this case, color measurement is carried out by the CCD in exactly the same manner as in the prior art, and the auxiliary measurements mainly serve to resolve ambiguities.
In the method according to the invention, the photodetector should advantageously, but not necessarily, be sampled at a frequency fp, which is an integral multiple of the frequency fc, of reading an entire intensity profile from the sensor array.
Another aspect of the invention also provides a method of determining the profile of a target surface, comprising the steps of scanning the target surface with an incident light beam; forming a beam of light reflected from said target surface; directing said reflected beam onto a sensor array to create an intensity profile signal; sampling said intensity profile signal at a first sampling rate to create a first set of data points representing the intensity distribution on said array in each sampling period 1/fc; directing at least a portion of the reflected beam onto photodetector means to obtain an output signal representative of the intensity of said reflected beam; sampling said output signal at a second sampling rate fp significantly higher than said first sampling rate fc to create second data points representative of intensity in each sampling period at said second sampling rate; and deriving from said first set of data points a third set of data representative of the relative position of said reflected beam on said sensor array as said beam scans said surface while using said second data to provide additional information at a higher resolution about the appearance of the surface within each sampling period at said first sampling rate.
Each profile measurement is the time integral of the reflection from the surface, over one cycle of the first sampling rate on the CCD. Oversampling of the returned intensity measured by the photodetector permits the surface texture, i.e. color or intensity, to be determined at a resolution higher than the profile. In principle, the sum, suitably normalized, of the oversampled measurements should correspond to the intensity measured by the CCD in a single cycle. This sum can serve to resolve the ambiguities in the manner indicated above. Moreover, oversampled measurements give a spatial distribution of surface texture resolution better than the profile measurement in the direction of scan.
The oversampling of the intensity measurements can also be used to correct anomalies caused by the discontinuities in the reflectance of the surface. By having a measurement of the time distribution of the reflection during one cycle of surface point measurement, it is possible to detect the presence of such discontinuities in the portion of the illuminated surface during the measurement of a single point from the distribution of measurements in the oversampled measurements. It is thus possible to compensate at least partially for the effect from the estimated measurement.
It should be noted that the last two advantages, which are related to the use of oversampling, exist both for monochrome and color measurements, and apply not only to the hybrid system proposed above, but also to systems employing only a photodetector sensitive to the different wavelengths, such as that described in U.S. Pat. No. 5,708,498.
The main problem addressed by the invention is the wavelength ambiguity when only a single peak is present. The method resolves this problem with the addition of a photodetector cell. This addresses a very important problem that limits the use of color-separated peak technology. The combination of this technique with the oversampling principle also opens the door to the possibility of measuring color at a resolution greater than that of the profile, which could be interesting in modeling and creating shape and color models for display purposes, where the texture density required is generally greater than the needs for geometric measurement. Moreover, it is possible from the oversampled intensities to correct known errors arising from intensity discontinuities.
The invention further provides an apparatus for determining the color and profile of a target surface, comprising a light source for scanning the target surface with an incident light beam containing a plurality of component wavelengths; a lens for forming a beam of light reflected from said target surface; a beam splitter for splitting the reflected beam into one or more sub-beams corresponding to the component wavelengths of the reflected beam; a sensor array for sensing said one or more sub-beams and permitting the positions of said one or more sub-beams to be detected as said incident beam moves over the target surface; and a wavelength sensitive photodetector receiving at least a portion of the reflected beam to obtain data representative of the approximate wavelength composition of said reflected beam; whereby the color and profile of the target surface can be determined from the positions and shapes of peaks produced by said one or more sub-beams on said sensor array using said data representative of the approximate wavelength composition to resolve potential ambiguities in the results obtained from said sensor array.